Pressurized water nuclear reactors include inside the building of the reactor, a vessel containing the core of the nuclear reactor formed by combustible assemblies, generally of prismatic form, arranged vertically and in a juxtaposed way.
Inside the vessel of the reactor, are arranged internal pieces of equipment in particular ensuring support and maintaining the combustible assemblies of the core of the nuclear reactor.
The vessel of a general cylindrical shape, is formed at its lower portion with a convex bottom and includes an upper end which is closed during the operation of the nuclear reactor by a convex lid. This lid of the vessel is crossed by tubes having a vertical arrangement, called adaptors used for ensuring the passage of extenders allowing displacement of control clusters in certain assemblies of the core.
When the lid is laid on the vessel, it determines a gasket plane formed by a ring-shaped recess made on a flange of the vessel and by a ring-shaped shoulder with a shape matching said recess and made on the vessel lid.
The recess and the shoulder of the vessel flange and of the lid respectively, determine two substantially horizontal planar surfaces facing each other and between which two O-ring gaskets are slightly crushed when the lid is clamped on the vessel.
The lid is attached on the vessel of the reactor by pins which are screwed into the flange of the vessel and which pass through in smooth holes provided in a flange ring of the lid.
These pins are generally in a relatively large number and relatively close to each other, disassembling and reassembling the lid therefore requires unscrewing and screwing of all these pins.
Moreover, during the screwing and unscrewing operations, by means of a tension device, traction is exerted on the pins in order to put them into a pre-stressed condition.
The seal during operation of the reactor between the vessel and the lid is therefore achieved by both of the O-rings, most often in “Inconel” (registered trade mark) or in stainless steel plated with silver. These gaskets are placed in two concentric grooves provided on the planar surface of the shoulder of the lid.
These gaskets bear on the planar surface facing the vessel flange and the spans of these gaskets are machined to as to obtain a perfect surface which does not accept any mark, any flaw.
Tightening the lid with the pins slightly crushes the gaskets, the supporting surface of which is about 1 mm wide and generally of the order of 3 mm.
The recess and the shoulder of the vessel flange and of the lid, respectively, each include a vertical wall delimiting between them a gap after the laying of the lid on the vessel flange.
During operations for reloading a portion of the nuclear reactor, cold shut-down of the reactor is performed and the lid closing the upper end portion of the vessel is disassembled.
After removing this lid, a temporary lid is placed on the vessel by placing a temporary inflatable gasket on the edge of the recess of the flange of the vessel in order to ensure a temporary seal.
During the laying of the lid on the flange of the vessel after reloading the core of the nuclear reactor, the temporary inflatable gasket is removed after performing decontamination of the pool by means of a pressure cleaner.
At the beginning of the operation for tightening the pins, the lid is slightly lifted relatively to the vessel flange due to the force produced by the springs of the combustible assemblies and the clamping ring. So there is a play from about 2 mm to 3 mm between the gaskets positioned below the lid and the gasket span over the vessel flange.
It is found that after maintenance operations a leak was noticed upon closing a vessel of a nuclear reactor and this leak was justified by the probable presence of metal microchips or other particles over the span of the seal gaskets of the vessel flange when clamping pins of the lid on said vessel flange are tensioned by a machine for tightening and loosening the pins.
The formation of these microchips probably results from the friction of the tops of the threads of the pins on the wall of the smooth holes of the lid during the mounting of said pins on the vessel flange with the tightening and loosening machine.
Because of the withdrawal of the temporary inflatable gasket before the operations for screwing the pins, these microchips might have made their way right under the gasket span by passing through the gap made between the shoulder of the lid and the recess of the vessel and reached the span of said gaskets during the phase for screwing the pin.
Upon restarting the unit at a primary pressure of the order of 25 bars and at a temperature of the order of 30° C., a leak may thus occur at the gasket span.
In GB 877,277, an annular C-shaped gasket is known which is placed at the gasket plane formed by the ring-shaped recess made on the flange of the vessel and by the ring-shaped shoulder with a shape mating said recess made in the vessel lid. Each end side edge of the annular gasket is welded on a horizontal wall, of the lid and of the vessel flange respectively.
The annular gasket is welded after laying the lid on the vessel so that during the welding operation, microparticles or foreign bodies may make their way right under the gasket span by passing through the gap.
Further, during a new operation for loading a portion of the nuclear reactor, before removing the lid, at least one of the welding beads has to be removed in situ which considerably complicates the operations, increases the shut-down time of the reactor and generates potentially bothersome chips.